1. Field of the Invention
The present invention relates to an active matrix liquid crystal display (AMLCD), and more particularly, to an AMLCD having thin film transistors (TFTs) as switching elements.
2. Description of the Related Arts
FIG. 1 shows a conventional active matrix liquid crystal display. The conventional AMLCD includes two substrates(a first and a second substrates), in which a plurality of pixels are formed in a matrix array.
On the first substrate 3, pixel electrodes 4 are disposed at the intersections between gate bus lines 17 and data bus lines 15. Gate bus lines 17 are formed in the horizontal direction and include gate electrodes(not shown) branching off therefrom. The data bus lines 15 are formed in the vertical direction and include data electrodes (not shown) branching off therefrom. At the intersections between the gate bus lines and the data bus lines, TFTs 8 are formed which make electrical contact with pixel electrodes 4.
On the second substrate 2, a color filter layer 38 and a common electrode 37 are formed.
The first and the second substrates are bonded together with a space therebetween. The space between the substrates is filled with a liquid crystal material 40. Polarization plates 1 are formed on the outer surfaces of the substrates before the bonding. Reference numerals 11 and 11' in FIG. 1 represent transparent glass substrates.
The structure and the method of manufacturing the first substrate 3 according to the present invention is described in detail with reference to FIGS. 2 and 3. FIG. 2 is a plan view showing the structure of a conventional AMLCD and FIG. 3 is a cross-sectional view taken along the line III--III in FIG. 2.
The structure of a conventional AMLCD is described below. On a transparent glass substrate 11, a gate bus line 17 in the horizontal direction and a gate electrode 17a branching off therefrom are formed. The gate electrode may be anode-oxidized to improve insulating performance and prevent hill-locks on the surface. On the substrate 11 including the gate electrode 17a, a gate insulating layer 23 made of an inorganic material, such as SiN.sub.x or SiO.sub.2, is formed. A semiconductor layer 22 made of amorphous silicon (a--Si) is formed on a portion of the gate insulating layer 23 over the gate electrode 17a. On the a--Si semiconductor layer, ohmic contact layers 25 made of n.sup.+ a--Si are formed to be disposed a predetermined distance away from each other. On the surface including the ohmic contact layer 25, a data bus line 15 is formed in one direction. A source electrode 15a is formed to be connected to the data bus line 15. A drain electrode 15b is formed a predetermined distance away from the source electrode 15a. The source 15a and the drain electrode 15b form electrical contacts with the corresponding ohmic contact layers.
A protection layer 26, made of an inorganic material such as SiN.sub.x, is formed to cover the substrate including the source 15a and the drain electrode 15b. A pixel electrode 4 made of a transparent conductive material, such as indium tin oxide(ITO), is formed on the protection layer. The electrode 4 is electrically connected to the drain electrode 15b through a contact hole 31 formed in the protection layer 26.
Since the first substrate of the conventional AMLCD results in a TFT and bus lines with a stepped surface profile as shown in FIG. 4, the pixel electrode 4 needs to be formed a predetermined distance away from the gate bus line 17, data bus line 17, and the TFT. This stepped profile appears because an inorganic material, such as SiN.sub.x or SiO.sub.2, is used for the gate insulating layer 23 and the protection layer 26.
Moreover, these stepped TFT and bus lines cause problems in the manufacture of an AMLCD. In particular, when an alignment film is formed on the stepped surface, the initial orientation of the liquid crystal becomes inhomogeneous. This degrades the quality of the LCD because of rubbing defects at the stepped portion of the alignment film.
In order to overcome such problems, an organic material with high planarization property is used for the gate insulating layer 23 or the protection layer 26. Then, the rubbing defects are eliminated, and the reduction in the performance of the LCD can be prevented. Moreover, an improvement in the aperture ratio can be achieved, since the pixel electrode 4 can be formed to overlap the bus lines.
However, the introduction of the organic material in a TFT structure causes new problems. The ON-characteristic of the TFT becomes unstable. For example, the ON-characteristics curve shifts toward the negative gate voltage as shown in FIG. 5. This is due to charge traps at the surface of the semiconductor layer 22 contacting the organic layer. Accordingly, better solutions are needed to obtain good planarization while preventing ON-characteristic instability.